The experiment, led by a team at the University of Minnesota, the United States, in March, was the first randomized clinical trial testing hydroxychloroquine for the post-exposure prevention of COVID-19, according to the university.
Researchers enrolled 821 adults from the United States and Canada, who had come into contact with someone confirmed with COVID-19 for more than 10 minutes at a distance of six feet (1.83 meters) or less.
All the participants were randomly assigned to receive either hydroxychloroquine, or a vitamin placebo, for five days.
Over the next two weeks, against laboratory test and from clinical signs, researchers found that 49 of the 414, or 11.83 percent, given the medicine got the COVID-19, compared to 58 of the 407, or 14.25 percent, on the placebo.
The scientists said the difference of about 2.4 percentage points in favor of the medicine was not considered statistically significant, which means hydroxychloroquine has no benefit over placebo in COVID-19 prevention.
Side effects, however, were more common with hydroxychloroquine group than those on the placebo -- 40.1 percent versus 16.8 percent -- no serious ones were reported though.
The trial results has been published in the New England Journal of Medicine.
U.S. President Donald Trump has said he used it as a prophylaxis against COVID-19, but the Food and Drug Administration cautioned in late April against the use of it outside of the hospital setting due to the risk of cardiac rhythm problems.
As of 0100 GMT Thursday, confirmed COVID-19 cases have passed 6.3 million globally, with 383,318 deaths, showed the tally of Center for Systems Science and Engineering at Johns Hopkins University.
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